I have a mercury one and it works fine. Whatever the thermometer it should be constructed such that the mercury bulb, or electronic probe, cannot come in contact with the metal pan. The temperature of the pan should be different then the scalding sugar goo. Also, the fewer crevices for the scalding sugar goo to cool and harden in the better. Remember to calibrate it by sticking it in boiling water.

I had a mercury thermometer mounted inside a glass tube once and the inside would steam up making it impossible to read the temperature. Now I just have a thermometer mounted to a metal piece which works wonderfully.